Machining is still the most important way of forming and is fundamental for production processes of all kinds of technical products. A detailed analysis of machining is important for the optimization of production processes. Therefore, a measurement of forces and moments is performed during machining, and the resulting information is used to quantify the energy converted during machining, determine material characteristics, analyze the wear of tools and enable a design of machine structures/components according to needs.
The document EP0806643A2A discloses a device for measuring forces and moments. The device is shown schematically in FIG. 1. On a T-shaped mounting platform 8 is mounted a workpiece 4 that is machined by a tool 5. A stem of the mounting platform 8 is supported between two legs of a U-shaped stand 9. Between each leg of the stand 9 and the stem of the mounting platform 8 is arranged a piezoelectric load cell A, B. These piezoelectric load cells A, B are mechanically preloaded by a preloading screw 10. The preloading screw 10 passes through the linearly arranged piezoelectric load cells A, B, the stem of the mounting platform 8, and the legs of the stand 9. During machining of the workpiece 4, a unidirectional force F acts in the direction of the longitudinal dimension of the preloading screw 10. The unidirectional force F is split equally between the piezoelectric load cells A, B. The piezoelectric load cell A is unloaded by a force of +½ F while the piezoelectric load cell B is loaded by a force of +½ F. Each of the piezoelectric load cells A, B comprises piezoelectric transducers. The crystallographic orientation of the piezoelectric transducers is such that a force acting thereon generates electric polarization charges. The piezoelectric transducers in load cell A have opposite polarization directions than the piezoelectric transducers in load cell B. Thus, a sum of the electric polarization charges of the piezoelectric load cells A, B is proportional to the magnitude of the force F acting on the workpiece 4 and thus acting in the opposite direction on the tool 5 with the same magnitude.
A device for force and moment measurement of this type having the type designation 9119AA1 is commercially available from Kistler Group, details are described in a data sheet No. 9119AA1_003_060e-01.13. The device comprises four piezoelectric load cells of which two are arranged on each of the sides of the stem of the mounting platform. The piezoelectric load cells measure three orthogonal force components Fx, Fy, Fz of the force F. Electric polarization charges are picked up in a component-specific manner and transmitted via signal lines to a charge amplifier where they are electrically amplified and then fed to an evaluation unit where they are converted in an electrical voltage. The device is characterized by parameters such as measuring range, sensitivity, natural frequency, weight, and an overall base area. The measuring range of the device is in the range of −4 kN to +4 kN for the force components Fx, Fy, Fz. The sensitivity of the device is ˜26 pC/N (10−12 Coulombs/Newton) for the force components Fx, Fz and ˜13 pC/N for force component Fy. The natural frequency of the device is ˜6.0 kHz for force component Fx, ˜6.4 kHz for force component Fy and ˜6.3 kHz for force component Fz. It has a weight of 930 g and its overall base area is the product of respective sides measuring 109 mm by 80 mm.
However, the prior art device for force and moment measurement according to data sheet 9119AA1_003_060e-01.13 shows anisotropic sensitivity with respect to the different force components. Particularly, in a horizontal working plane of the mounting platform, the sensitivity of ˜26 pC/N of the force components Fx is twice as high as that of the force component Fy being ˜13 pC/N. Since machining is often performed in a rotationally symmetrical manner with respect to the horizontal working plane, an actual sensitivity of the device in the working plane is determined by the low sensitivity regarding force component Fy. Furthermore, it is desired to increase the natural frequency of the device since the lowest natural frequency Fx of ˜6.0 kHz restricts the use of the device to maximum spindle speeds of ˜60,000 rotations per minute.
It is the object of the invention to increase at least one of the parameters of sensitivity and natural frequency of a device for force and moment measurement.